1. Field of the Invention
This invention relates to a multi-channel compact camera system.
2. Description of the Related Art
Cameras are well known devices for capturing a visual image. With the advent of electronic image sensors, digital cameras started using image sensors such as a CCD image sensor or a CMOS image sensor. The use of an image sensor easily allowed the camera to store the captured image as digital information. The digital information could be analyzed separately or could be used to reproduce the visual image.
As the typical camera is used to capture a visual image as seen by the human eye, digital cameras were designed with that purpose in mind. Most cameras capture the spectrum of light that correspond to visible light, approximately 450 to 700 nm. In particular, these cameras capture three spectral bands, (1) red light, (2) green light, and (3) blue light. If light from these three spectrums are captured and reproduced, the resulting image would closely mimic the image as seen by the human eye.
To make digital cameras capture color, they are made to absorb light in a way that's similar to the way our eyes do. That is, they contain arrays of photosites that sense red, green, or blue light. Traditionally this is done in one of two ways: 1) Using a single image sensor which incorporates a Color Filter Array (CFA); or 2) Using a three image sensor configuration with a multiple channel beam splitting component that separates images into discrete red, green and blue channels using conventional filters. Using a single image sensor which incorporates a CFA will generally have poorer resolution than multiple image sensors. However, a multiple image sensor configuration will increase the size and cost of the camera. It would be advantageous to have a camera system that maximizes the compactness of the device while maintaining the quality of data to suffice for the purposes of the camera.
It is known that the human eye is most sensitive to green light. In addition, the human eye's perception of resolution peaks in the green portion of the spectrum and is much more sensitive to brightness (luminance) variations than to color (chrominance) variations. To mimic this property of the eye, digital cameras will employ CFAs that will allow more green light to filter through the CFA than red and blue. One such popular CFA (RGGB filter), will allow twice as much green light to filter through than red light or blue light. This results in twice as much resolution from the green channel of a typical single-chip camera, than from either red or blue one. This chroma subsampling (G:B:R sampling ratios of 2:1:1) is the practice of encoding images by implementing less resolution for chroma information than for luma information, taking advantage of the human visual system's lower acuity for color differences than for luminance.
CCD and CMOS image sensors are sensitive to light outside of the visible spectrum, specifically the near infrared (“NIR”) portion of the spectrum (approximately 700 to 900 nm). Because of this sensitivity, color RGB cameras employ NIR blocking filters to eliminate color cross-talk or color distortion in the RGB Image.
The NIR portion of the spectrum does, however, have significant information content. Some specialized RGB camera replace the NIR blocking filter with a blue blocking filter to create a Green-Red-Near IR (GRN) image capture or Color-IR image capture device. Because the blue CFA material transmits both blue and NIR light, this filter change results in collecting NIR light in pixels originally purposed to collect blue light. In this configuration the camera captures three spectral bands with a G:R:N sampling ratio of 2:1:1 and when the bands are combined they generate useful false color images for assessment of information content.
Each individual spectral band, however, has significant image information content of its own. Also, ratios of individual bands can be used to extract additional information content not easily obtained from individual bands. Unfortunately, the low sampling ratios of the B, R and N bands in conventional camera configurations result in low resolution band content and the ability to assess information content from these bands is diminished.
Current color cameras are extremely wasteful with light. To capture color images, a filter or series of filters needs to be placed over each pixel of an image sensor that permits only particular colors of light. Due to subsampling of a RGGB pattern only 50% of incoming green light and 25% of red and blue light is available to be captured. Furthermore, because of the attenuation of the red, green and blue filters only half of that available light can reach the light sensing elements of the sensor. The net result is that roughly 25% of all green light and 12.5% of all red and blue light actually reach the light sensing portion of the sensor.
It would be advantageous to have a camera system that maximizes the compactness of the device while capturing four simultaneous spectral bands (red, blue, green and NIR) with minimal spectral crosstalk while maximizing the quality of data (sampling density and light transmission) and, if desired, maintain a R:G:B:N sampling ratio of 1:1:1:1 to suffice for the purposes of maximum image information content assessment from the camera.